PLATE XI. 



Fig. I. Tinguaite. Crossed Nicols. X ss- Crystal of nephe- 

 lite in ground-mass of segirite and feldspar. 



Fig. 2. Tinguaite. Crossed Nicols. X 33. The phenocryst is 

 of weathered and corroded feldspar. The replace- 

 ment by secondary minerals, beginning at the periph- 

 ery of the crystal, is first by a zone of feldspar rods, 

 and intergrown pyroxene needles. The dark zone is of 

 analcite. The colorless central portion is orthoclase. 



(304) 



